Aequalis Commutationem (Equivalent Exchange)

This spell allows the caster to cast greater wish as a 3rd level spell but they must give something in exchange. Typically the only thing viewed as being a worthy trade to the gods for someone who cannot cast wish naturally to gain this awesome power is the giving of the casters own life, or a massive sacrifice of souls.  So this spell is rarely used and considered by most to be forbidden dark magic or a devils contract.  Knowledge of this ritual is a closely guarded secret.

must be cast as a ritual   

Caster encircles them self with a combination of ash and their own blood. Standing in the center of the circle, the caster will swallow the gemstone, then take the arcane focus and destroy it by any magical means so long as it is done by the caster while both they and the focus are within the circle. then recite and incantation and the gemstone will glow allowing it to be seen through the skin or scale of the caster. take a golden blade and plunge it into the point where the crystal lies and remove the crystal from the body. once the caster has cut the crystal out of them self they must say the final incantation then smash the glowing crystal inside the circle as they state their wish. Once the ritual is complete the caster is ripped apart and their consciousness is brought before a god (the dm or whichever being the dm sees fit to grant your wish) and discuss the details of your wish.

The ritual itself is meant as a deceleration to all gods, demons, devils, and arch-viles that exist and can be heard by all you state your wish and invoke any power that would grant it to use the force of your soul to make your desire a reality. due to the magical force created by the soul when casting the spell if these great beings ignore your cry then the force will be such that the fabric of reality shatters and all around you is destroyed so the gods dare not ignore, even the darkest of beings hides a twinge of fear at the raw power produced by this act. and for your sacrifice and power in the casting of this spell you wish is almost certain to be carried out in a satisfactory manner. The spell has generated tremendous magical energy using the casters very soul as fuel but also bound it to the casters wish so any attempt to harness it for something else would surely annihilate anyone foolish enough to try. due to the volatile and unpredictable nature of this spell and the nature and strength of the energy that it produces, being enough to erase a god from the heavens, it is forbidden and even the knowledge of the rituals existence is a fiercely guarded secret.  

This version of wish is a direct request of the dm to do something like, resurrect multiple characters (cannot be the character that casts this spell), give an item to the remaining player characters, do something in the lore for pc or relating to the characters backstory, etc. 

the stress of casting this spell completely destroys the caster, this character is dead and cannot be brought back by anything(unless a dm makes an exception for a normal casting of wish or a high level magic item) but it should not be easy to do if even possible at all.

Wish is the mightiest spell a mortal creature can cast. By simply speaking aloud, you can alter the very foundations of reality in accord with your desires.

The basic use of this spell is to duplicate any other spell of 8th level or lower. You don't need to meet any requirements in that spell, including costly components. The spell simply takes effect.

Alternatively, you can create one of the following effects of your choice:

• You create one object of up to 25,000 gp in value that isn't a magic item. The object can be no more than 300 feet in any dimension, and it appears in an unoccupied space you can see on the ground.
• You allow up to twenty creatures that you can see to regain all hit points, and you end all effects on them described in the greater restoration spell.
• You grant up to ten creatures that you can see resistance to a damage type you choose.
• You grant up to ten creatures you can see immunity to a single spell or other magical effect for 8 hours. For instance, you could make yourself and all your companions immune to a lich's life drain attack.
• You undo a single recent event by forcing a reroll of any roll made within the last round (including your last turn). Reality reshapes itself to accommodate the new result. For example, a wish spell could undo an opponent's successful save, a foe's critical hit, or a friend's failed save. You can force the reroll to be made with advantage or disadvantage, and you can choose whether to use the reroll or the original roll.

 
You might be able to achieve something beyond the scope of the above examples. State your wish to the GM as precisely as possible. The GM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong. This spell might simply fail, the effect you desire might only be partly achieved, or you might suffer some unforeseen consequence as a result of how you worded the wish. For example, wishing that a villain were dead might propel you forward in time to a period when that villain is no longer alive, effectively removing you from the game. Similarly, wishing for a legendary magic item or artifact might instantly transport you to the presence of the item's current owner.

The stress of casting this spell to produce any effect other than duplicating another spell weakens you. After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d10 necrotic damage per level of that spell. This damage can't be reduced or prevented in any way. In addition, your Strength drops to 3, if it isn't 3 or lower already, for 2d4 days. For each of those days that you spend resting and doing nothing more than light activity, your remaining recovery time decreases by 2 days. Finally, there is a 33 percent chance that you are unable to cast wish ever again if you suffer this stress.